This invention relates generally to duct static pressure determination in air distribution systems and more particularly to making such a determination independent of duct pressure sensors.
The utilization of variable air volume (VAV) air distribution systems to supply conditioned air from a central source thereof to offices, school rooms, and other similar spaces or areas in multi-room buildings has become increasingly more prevalent. Such VAV systems generally furnish varying volumes of air, at constant temperatures, into a space in accordance with the space or zone demands. The flow of conditioned air from outlets or terminals is generally regulated by operation of suitable damper means controlled by a thermostat sensing the temperature of the space being conditioned. Thus, as the temperature of the space deviates to a greater degree from a predetermined setpoint, the damper opens more and a greater quantity of conditioned air is discharged into the space. Conversely, when the temperature in the space being conditioned approaches the setpoint, the system decreases the air volume to the space depending upon the deviation of the space temperature from the setpoint. In U.S. Pat. No. 4,756,474 assigned to the same assignee as the present invention, there is described a pneumatic controller for a duct pressure powered air terminal unit having a volume controller which receives two pressure signals, whereby the controller bleeds one pressure signal so as to control the inflation of a bellows or bladder to thereby modulate the terminal unit to maintain a desired volume air flow through the unit, and bleeds the second pressure signal so as to maintain at least a minimum flow through the unit. The above-identified controller is an improvement over U.S. Pat. No. 4,120,453 which describes a three-way valve controller having two pressure regulators and a bleed type thermostat which provide four input signals to the three-way valve thereby providing a single pressure signal to the inflatable bellows.
Since the air terminals in the aforementioned patents are powered by static pressure in the duct and continuously fill and bleed the bladder to maintain a desired space temperature, the static pressure in the duct is continually changing. Further, under normal operating conditions the air terminals closer to the central air source have a high static pressure than air terminals more distant from the central air source. These continually changing duct static pressures make it extremely difficult to maintain proper operation of the air supply system. Conventionally, the statio pressure in the duot is maintained at a predetermined setpoint by a single pressure sensor which is located somewhere in the main duct line. The pressure sensor sends an input to a fan control such as a variable speed drive of an air handler unit to cause the speed to increase, decrease or remain constant as required, and can also send a signal to a control panel or building supervisor. Thus, if the pressure at the sensor location is too high, a "decrease" signal is sent and, if it is too low, and "increase" signal is sent. The location of the sensor is carefully chosen in order that the pressure sensed is most closely representative of the overall system pressure distribution over the whole load range. Ordinarily, the sensor location chosen is about two thirds of the way down the main duct as measured by distance or total pressure drop. So, even if the pressure sensor is located at the point where the sensor will give the "best" representation of the overall duct pressure distribution at design flow, when there is off design flow, or when there is leakage or blockage in the duct, the single sensor provides only a single reading. As a result, the sensor may be inaccurate in its representation of duct pressure distribution under off-design conditions as well as those due to problems in the duct.
Independent of how well placed it is, a single sensor provides a single reading and it is not an average or otherwise influenced by measurements taken at other locations. Because of this, a single reading cannot give a service technician a good view of what is required in the duct under all conditions. Since a full view is not provided, the service technician is required to measure the flow from various terminals or enter the duct to obtain additional pressure readings in order to troubleshoot any duct static pressure problems. Thus there is a clear need for a method and apparatus to determine duct pressure at various locations in the duct system in order to trouble shoot duct static pressure problems without the use of a pressure sensor at each location, but using existing mass air flow sensors associated with air terminals. However, no previously proposed air distribution system has incorporated any means to determine duct pressure at various location without a plurality of pressure sensors at such locations.